This invention relates to an automatic tracking servo control system for use in reproducing apparatus wherein at least one rotary transducer scans successive parallel record tracks on a movable record medium to reproduce information therefrom and, more particularly, to such a system wherein proper tracking control is attained even if certain parameters which had been present during recording are changed during reproduction.
As is known, a wide-band signal can be recorded advantageously on a movable record medium by use of one or more rotary transducers, or heads, which scan parallel tracks across the record medium. One example of such a wide-band signal is a video signal, for example, a composite color video signal, and one example of recording apparatus of the aforementioned type is a video tape recorder (VTR). In a typical VTR, the recording head or heads are rotated at a relatively high rate while the tape is moved so as to record skewed, parallel tracks. The recorded information is reproduced by scanning such record tracks with one or more rotary playback heads. As is conventional, the same head which is used to record signals also can be used to reproduce such signals.
It is appreciated that, in order to obtain an accurate reproduction of the signals which are recorded in parallel tracks, a proper relationship must exist between the rotary head, or heads, and the track being scanned. That is, the head or heads must be rotated at the same speed during a reproducing operation as during a recording operation; and the magnetic tape likewise must be transported at the same speed during both a recording and a reproducing operation. Still further, each head must be positioned so as to sweep across the particular record tracks which is being scanned in proper alignment with that track. To achieve this, a typical VTR includes a tracking servo system.
In a reproducing operation, the aforementioned tracking servo system is operated so as to control the relative movement between the rotary head or heads and the magnetic tape. Generally, the head or heads are rotated at a fixed, precisely controlled speed. Hence, the servo system is effective to control the speed at which the tape is transported. This tape speed is adjusted, if necessary, in order to bring the heads into exact alignment with the tracks being scanned. To accomplish this, position pulses are generated when the head or heads rotate into predetermined positions relative to the tape. For example, a position pulse is generated when the head first rotates into magnetic relation with the tape. Also, a series of control pulses, which had been recorded in predetermined, spaced position along the length of tape, are reproduced and used in conjunction with the position pulses to control the relative movement, and thus the position, of the heads and tape. Typically, each control pulse is recorded so as to be spaced a predetermined amount from a corresponding record track. When the control pulses are reproduced, each exhibits a predetermined phase shift with respect to a position pulse. Consequently, a proper tracking relation is achieved if this predetermined phase relation is maintained. The tracking servo system functions to adjust the transport speed of the tape in order to maintain this phase relation.
A tracking servo system of the type described above generally operates satisfactorily if the same recording machine that had been used to record the signal information also is used to reproduce that information. However, if different machines are used for recording and reproduction, or if various operating parameters change from the recording operation to the reproducing operation, the tracking servo system might perform less than satisfactorily. For example, if different machines are used, the speed of the rotary heads in one machine may differ from that in the other. Also, if the respective machines are of the so-called "helical-scan" type, the length and angle of the respective head scanning traces may differ. Still further, the recorded tape may stretch or shrink prior to the time that the information thereon is reproduced. These factors, as well as others not mentioned herein, will impede the satisfactory operation of the tracking servo system so as to hinder a proper servo "lock".
To account for the aforementioned factors, a manual adjustment may be provided in order to manually change the phase relation between the reproduced control pulses and the generated position pulses. For example, a potentiometer may be provided to adjust the phase of the control pulse. That is, in order to obtain a servo locked condition, the reproduced control pulse may be delayed by an adjustable amount so as to exhibit a new phase relation with respect to a position pulse. It is this new phase relation which results in the aforementioned servo lock condition. Such a manual adjustment may require great care from a skilled technician. It may be exceedingly difficult to effect the desirable manual adjustment by the user of, for example, a home-entertainment VTR.
An improved automatic tracking servo control system is disclosed in our copending application Ser. No. 940,269, filed Sept. 7, 1978. In that system, the aforenoted difficulties are overcome by automatically adjusting the phase of the control signals as a function of the difference between the strength of the signals reproduced during one interval and the strength of the signals reproduced during a preceding interval. If the earlier-reproduced signal strength is greater than the later-reproduced signal strength because of an increase in the tracking error, the direction in which the control signal has been phase shifted is changed by an incremental amount. However, if the later-reproduced signal strength is greater than the earlier-reproduced signal strength because the scanning traces of the rotary heads and the record tracks are brought toward alignment so as to reduce the tracking error, then the direction in which the control signals are incrementally phase shifted is maintained. One aspect of the above system is to shift the phase of the control signals so that the tracking of the rotary head hunts above the correct phase thereof so as to change its tracking condition in search for an improved condition.
The present invention is directed to a further improvement in an automatic tracking servo control system.